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An Oncoprotein from the Plant Pathogen Agrobacterium Has Histone Chaperone–Like Activity^[W]

^a Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
^b Graduate School of Natural Sciences, Nagoya City University, Mizuho, Nagoya, Aichi 467-8501, Japan
^c College of Bioscience and Biotechnology, Chubu University, Kasugai, Aichi 487-8501, Japan
^d Faculty of Bioresource Sciences, Akita Prefectural University, Nakano-Aza Kaidobata, Shimoshinjo, Akita 010-0195, Japan
^e Institut de Biologie Moléculaire des Plantes, Centre National de la Recherche Scientifique, Unité Propre de Recherche 2357, 67084 Strasbourg, France
^f Division of Biological Science, Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan

¹ Address correspondence to yas{at}bio.nagoya-u.ac.jp.

Protein 6b, encoded by T-DNA from the pathogen Agrobacterium tumefaciens, stimulates the plant hormone–independent division of cells in culture in vitro and induces aberrant cell growth and the ectopic expression of various genes, including genes related to cell division and meristem-related class 1 KNOX homeobox genes, in 6b-expressing transgenic Arabidopsis thaliana and Nicotiana tabacum plants. Protein 6b is found in nuclei and binds to several plant nuclear proteins. Here, we report that 6b binds specifically to histone H3 in vitro but not to other core histones. Analysis by bimolecular fluorescence complementation revealed an interaction in vivo between 6b and histone H3. We recovered 6b from a chromatin fraction from 6b-expressing plant cells. A supercoiling assay and digestion with micrococcal nuclease indicated that 6b acts as a histone chaperone with the ability to mediate formation of nucleosomes in vitro. Mutant 6b, lacking the C-terminal region that is required for cell division–stimulating activity and interaction with histone H3, was deficient in histone chaperone activity. Our results suggest a relationship between alterations in nucleosome structure and the expression of growth-regulating genes on the one hand and the induction of aberrant cell proliferation on the other.